Electroluminescent structure with improved protective transparent film



F 1967 e E. CLOCK ETAL 3,3

ELECTROLUMINEESCENT STRUCTURE WITH IMPROVED PROTECTIVE TRANSPARENT FILM Filed Dec. 22, 1961 burneaou/ area 0 /00 200 300 400 500 600 Hours a) 35-96 70 re/a/ive hum/b0)? 0/ 987a 1007-? IOO 6 INVENTORS.

Gera/aEC/ok 5 James/(- RI'e/(e By Edward S/ro/was 5 8 Z 7 F291 flTTORNEY United States Patent O 3,305,745 ELECTROLUMINESCENT STRUCTURE WITH IM- PROVED PROTECTIVE TRANSPARENT FILM Gerald E. Clock, James K. Rieke, and Edward Stroiwas,

Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Dec. 22, 1961, Ser. No. 161,648 3 Claims. (Cl. 313-108) The invention pertains to improved electroluminescent panels and lamps.

Electroluminescence refers to the emission of light by the excitation of particles of a selective material (known as a phosphor) substantially uniformly dispersed in a dielectric matrix when subjected to oscillating or alternating electric current.

The discovery of the principle of electroluminescence has given rise to its application in electroluminescent structures. Such structure basically comprise an intimate mixture of a phosphor dispersed in a dielectric material having adhered to one side an electrically conducting base (usually of metal), to comprise a phosphor plate, and adhered to the other side a light-transmitting electrically conductive base (usually of metal), to comprise a phosphor plate, and adhered to the other side a light-transmitting electrically conductive film, each of said base and light-transmitting films being provided with electrical terminals secured thereto. Such structure, resembling a sandwich arrangement, will usually be referred to herein as an electroluminescent panel. Protection of the panel by some exterior coating is necessary if the panel is to resist deterioration due to moisture for any appreciable time. An electroluminescent structure consisting essentially of a phosphor plate and light-transmitting electrically constructive film (i.e., the electroluminescent panel),

provided with at least one external protective coating over at least the light-transmitting face and adapted for practical use will usually be referred to hereinafter as an electroluminescent lamp.

Numerous practical uses have been made of electroluminescent lamps. The most extensive uses heretofore have been as identifying and directing plates and markers in dark passageways and chambers and in instrument circuitry panels which are not subjected to protracted periods of exposure to severe climatic conditions.

Although some use has been made of electroluminescent lamps for out-of-doors markers, signs, and the like, such use has been limited because the deterioration thereof in use under severe conditions of moisture, extremes of heat and cold, and the suns rays has been more rapid than can usually be tolerated.

In our copending application Serial No. 161,485, now abandoned, filed concurrently herewith, there is described an electroluminescent structure consisting essentially of a phosphor plate-tin oxide type panel protected against attack by moisture either by a single adherent film of the graft copolymer of a predominant proportion of a C -C polyolefin and a corresponding lesser roportion of acrylic acid or methacrylic acid or such film employed as a first or subfilm and supplemented by a second adherent external film of a theretofore known protective film against deterioration by ultra-violet light.

We have now discovered that the random copolymer of a predominant proportion of a C -C olefin and a corresponding lesser proportion of acrylic acid or methacrylic acid, when employed as a first or subfilm and supplemented by second adherent film exterior thereto, provides satisfactory protection against deterioration of electroluminescent panels.

The invention, accordingly, is an electroluminescent structure consisting of a phosphor panel (a pair of electrically conducting plates or films, at least one of which 3,305,745 Patented Feb. 21, 1967 ICC is transparent, provided with electrical terminals and a phosphor-dielectric layer therebetween) and a protective multi-ply resinous outer layer consisting of a first or subfilm or underply of the random copolymer of ethylene or propylene and acrylic acid or methacrylic acid adhered to at least the transparent conducting face of the panel and making sealing engagement with the edges thereof, and a second or outer and supplemental protective transparent ply or film adhered to the underply.

The invention is not to be confused with the use of a random copolymer of an olefin and acrylic or methacrylic acid as the sole moisture barrier film. Electroluminescent structures employing this type of moisture protection have not been fully satisfactory. It is also not to be confused with the use of known adhesives in general to adhere moisture protective films to electroluminescent panels. The discovery of the superior protective properties afforded electroluminescent panels by a multi-ply layer, wherein the random copolymer of the olefin and acrylic or methacrylic acid is the underlying adhesive film, in the face of knowledge that the random copolymer alone was unsatisfactory and also that such random copolymer used as the exterior film, with heretofore known adhesives as the underfilm were unsatisfactory, was unexpected.

FIGURE 1 of the drawing represents schematically a diagrammatic view of a cross-section of an electroluminescent lamp of the invention comprising phosphor panel 1 composed of phosphor-dielectric layer 2 and metal base 3 and transparent conductive film 4 adhered to the bottom and top surfaces of layer 2, respectively, electric terminals 5 and 6 afiixed to base 3 and film 4, respectively, subfilm protective barrier 7 of the random copolymer of a C -C olefin and acrylic or methacrylic acid, and exterior protective film 8 of a low moisture penetrability, e.g., polystyrene, styrene-methylmethacrylate copolymer, or glass.

FIGURE 2 shows graphically the moisture resistant properties attained by the practice of the invention in contrast with such properties when the invention was not practiced.

The random copolymer useful in the practice of the invention may be prepared by reacting a major proportion of ehtylene or propylene with a minor proportion of acrylic or methacrylic acid at a pressure of at least about 500 atmospheres and a temperature of at least about C. in the presence of a polymerization free radical-promoting catalyst, e.g., oxygen, benzoyl peroxide, or teltiary butylperoxide, in a suitable reaction vessel. Such method is described in US. 2,200,429 and subsequent improvement patents thereover, and is often adhered to as the Imperial Chemical Industries (I.C.I.) method.

Ethylene and acrylic acid are the preferred monomers to employ. Preferred proportions are between about 70 and 98 parts by weight of ethylene and correspondingly between about 30 and 2 parts of acrylic acid.

The invention lends itself well to the use of a third or additional coating or film, adhered to the exterior surface of the second moisture-barrier coating for the purpose of screening or filtering out a substantial portion of ultraviolet light incident thereon. Polyesters are preferred for this purpose.

The term, electroluminescent panel, is used herein as briefly described above, to refer to a phosphor plate, usually a metal base having adhered thereto a solidified layer of a fused intimate mixture of a phosphor, commonly ZnS activated by Mg, Se, Cu, or Zn, and Cl, and a dielectric material such as a ceramic or glass and which in turn, has adhered thereto, a thin transparent electrically conducting film, e.g., SnO For practical purposes, such panel when in use must be protected against the attack of moisture and other airborne substances by a tightly sealed adherent film, as aforesaid. Such a film, of course, must transmit light. It is usually applied as an encapsulant, i.e., as an enveloping sheath for the entire panel, although it may be employed over the lighttransmitting face only, as an adherent film which is sheets, 17 mils in thickness, composed of polyethylene, were then placed in aligned positions over the sheets of the random copolymer. The thus assembled panels, having the protective film of sheets successively adhered tightly sealed at the abutting contacting edges of the base, 5 thereto, were then placed between the platens of a heated phosphor dielectric layer, and transparent electrically press and subjected to a pressure of about 100 p.s.i., at conducting film assembly, a temperature of about 175 C., for about 3 minutes.

The invention is most conveniently carried out by The structures so made were then removed. The multiplacing either a single film or the olefin-acrylic or rnethply layers thus provided on both the back and the front acrylic acid random copolymer on the light-emitting face of the panel were observed to be firmly adhered to the of the electroluminescent panel or preferably by placing entire contacting surface of the panel and to form tight one sheet thereof on top and another sheet thereof besealing engagement with each other at the abutting edges low the panel, allowing the sheets to extend beyond the of the panel. The structure constituted an electrolumicdges of the panel at least about to provide an overnescent lamp ready for installation. lapping margin; laying the sheet (or pair of sheets) of Example 2 the outer protective film thereover in substantial alignment therewith; putting the thus assembled panels and A second 2.75-lI1Ch diameter circular electrolumisheets between heamontmned platens f a press d nescent panel was encapsulated similarly to the procedure applying a pressure of between about 50 and 200 psi. followed in Example 1 except that the P y Sheets at a temperature of between about 120 and 240 C., 20 of random copolymer of hy and acrylic field Were for a period of between about 0.5 and 12 minutes. The 2 hrlls rhlek and lhe Outer pelyethylerle films Were 13 pressure, temperature, and period of time usually emmlls Thickployed are between about 100 and 150 p.s.i., between The lamps of both Examples 1 and 2 Were tested for about 90 d 120 C and between 2 d 4 minutes resistance to deterioration due to moisture by connecting respectively. the terminals thereof in a 160 volt, 60 cycle A.C. circuit To illustrate the practice of the invention, Examples and Placing them in a humid warm air Oven at between 1 and 2 were run. These examples are to be construed and 98% relative humidity and between 98 and as representative of the invention and not as limitations and Periodically exarhlhlhg the lamps for h g terioration and impairment of performance.

The random copolymer employed in the examples For P p of comparison, test p designated was prepared as follows: an autoclave reactor suitable A to D Were made Which either eerllalrled r10 Preteellve f hi h pressure, hi h temperature reactions (of h type film about the electroluminescent panel or were provided m l l d i h {Cl h d was dj d with a protective layer which was not in accordance t a temperature f 224 C d a pressure of 15 000 with the invention and were subjected to the same test p.s.i.g. Into the autoclave reactor, while maintaining the as Examples 1 and above conditions, there were fed ethylene at a rate of The table, infra, Sets out the Inert important facts about 33.0 parts by Weight per hour, acrylic acid at a concerning the protective moisture barrier film used and rate f about 134 parts Per hour, and ditertiarybutyl the test results obtained by examination of the test samperoxide at a rate of 0,04 part per hour The ensuing ples. The per cent failure was ascertained by measuring ti d d the random copolymer f ethylene 40 the dimensions of the burned-out portions of the lightd acryhc id containing about 10% li id at a emitting face of the samples, calculating the total area rate f 65 parts per 1 of the burned out portions, and dividing the sum of such Example 1 area by the original light-emitting area.

' FIGURE 2 of the drawing graphically shows the A 2.75-inch diameter circular electroluminuescent superiority of the invention over the comparative runs panel, consisting of a thin steel base, a phosphor-diin protecting the electroluminescent panels, while in use, electric layer compound of activated ZnS substantially against the attack of moisture.

TABLE Test Results After Specified Number of Hours in Warm Humid Air 1 Moisture Barrier Protective Film Employed, Full Expressed in Percent Failure 2 Test No. Encapsulation Examplel 3.5mil-thieksubfilmof R. EthrAA and outer17-n1i1 1 15 30 35 thick protective P.E.- adhered thereto. Example2 2-mil thick subfilm of R. Etl1.-AA and outer 13-mil 1 15 30 40 thick film or RE. Comparative Runs:

A. No moisture-barrier provided .3 40 100 B Moisture-barrier of single l2-mil thick layer of RE. 1 75 0.95 density and melt index of 5. C Moisture-barrier of single layer of R. Eth.AA 12-1nil 1 70 thick, melt index of 2.5. D "do 20 90 100 1 The air was controlled at between 95-98% relative humidity and between 98 and 100 F.

2 The percent failure was calculated by ascert spots (burned out portions) and dividing that ar aining the area of dead ea by the original lightf the copolymer transmitting area and converting to percent.

3 Random copolymer of about ethylene and about 10% acrylic acid. 4 Polyethylene.

Reference to the examples of the table and the contrast thereof to the comparative runs, representaitve of heretofore better known methods, clearly shows the superiority of the moisture barrier protection afforded by the multi-ply layer consisting of a su'bfilm of a random copolymer of an olefin and acrylic acid or methacrylic acid, having adhered thereto an exterior film of transparent materials of known resistance to penetration of moisture. Although the reason for the excellent results obtained when the specific random copolymer is employed in combination with a second moisture protective film adhered exterior thereto is unexpected not clearly understood (in view of the unsatisfactory performance of the random copolymer or the sole moisture protective film or as the exterior film employing known adhesives), the excellent performance thereof is definitely a marked improvement in the art of making of electroluminescent lamps.

Further examples were run employing the random copolymer of ethylene and acrylic acid in varying thicknesses as the underlying adherent protective film having transparent moisture barrier films of thin glass (microsheet) adhered exterior thereto were run in accordance with the invention. The moisture protection obtained was superior to that obtained when such glass was employed either singly or in conjunction with other heretofore known adhesives. Similarly, polystyrene and polymethylmethacrylate serve satisfactorily as the second moisture-protective film. It is recommended that the third film having a filtering effect on ultraviolet light, e..g., Mylar, be also provided exterior to the glass, polyolefin or the other moisture-barrier.

Having described the invention, what we claim and desire to protect by Letters Patent is:

1. In an electroluminescent structure comprising an assembly composed of an electrically conducting solid base, a layer of an intimate mixture of a phosphor and a dielectric material having one -face in firm contact therewith, an electrically conducting light transmitting coating in firm contact with the opposing face of said layer, the improvement consisting essentially of a protective transparent film of a copolymer consisting essentially of an olefin containing from 2 to 3 carbon atoms per molecule randomly copolymerized with an unsaturated acid selected from the class consisting of acrylic acid, methacrylic acid and mixtures thereof, encapsulating said assembly, and adhered firmly thereto, a second transparent film exterior to and firmly adhered to the first film, said second film being selected from the class consisting of high density polyethylene and polypropylene.

2. The electroluminescent structure of claim 1 wherein said copolymer is the copolymer of ethylene and acrylic acid in proportions of between 70 and 98 percent ethylene and 30 to 2 percent acrylic acid.

3. The method of making an electroluminescent lamp which exhibits long life in use in an atmosphere containing airborne corrosive chemicals and moisture which consists essentially of encapsulating an electroluminescent panel previously prepared by intimately admixing a phosphor consisting of ZnS, activated .by at least one element selected from the class consisting of Mg, Se C-u, Zn, and

Cl, dispersed in a fused dielectric material, and the resulting mixture formed into a layer having adhered to one face thereof an electrically conducting base plate provided with an electric terminal and having adhered to the opposing face thereof a thin transparent electrically conducting film provided with an electric terminal of opposite charge to that of the base plate to make a sandwich-like panel, placing over and under and in contact with the opposing faces of said panel sheets of a random copolymer of an olefin containing from 2 to 3 carbon atoms per molecule and an unsaturated acid selected from the class consisting of acrylic acid, methacrylic acid and mixtures thereof in a weight proportion of between and 98% of the olefin and between 30% and 2% of the acid, and placing over and under and in contact with the random copolymer sheets and in substantial alignment therewith, sheets selected from the class consisting of a high density polyethylene and high density polypropylene, all of said sheets extending beyond the edges of said panel a distance of at least about inch to provide an overlapping margin, placing the panel having said sheets positioned successively exterior there-to between the platens of a heated press and subjecting the assembly so made to a pressure of between about 50 and about 200 p.s.i. at a temperature of between about and 200 C. for about 0.5 to about 5.0 minutes to produce a unified structure, having a firm bonded adherence between the under sheets and the outer sheets and between the under sheets and said panel and having formed a fused peripheral seal about the edge of said panel to provide a lamp having a multpiple encapsulating protective sheath, and removing the lamp so made from the press.

References Cited by the Examiner UNITED STATES PATENTS 2,932,323 4/1960 Aries.

3,030,542 4/1962 Knochel et a1 313-108.1 3,037,138 5/1962 Moison 313108.1 3,055,784 9/1962 Roed'el 162252 3,132,120 5/1964 Graham et a1. 26086.7 X 3,157,709 11/1964 Hoch et al 260-878 X OTHER REFERENCES Modern Plastics, vol. 35, Breskin Publications, N.Y., 1958, pages 94-95.

JAMES W. LAWRENCE, Primary Examiner. ARTHUR GAUSS, Examiner.

GEORGE N. WESTBY, C. R. CAMPBELL, R. JUDD,

Assistant Examiners. 

1. IN AN ELECTROLUMINESCENT STRUCTURE COMPRISING AN ASSEMBLY COMPOSED OF AN ELECTRICALLY CONDUCTING SOLID BASE, A LAYER OF AN INTIMATE MIXTURE OF A PHOSPHOR AND A DIELECTRIC MATERIAL HAVING ONE FACE IN FIRM CONTACT THEREWITH, AN ELECTRICALLY CONDUCTING LIGHT TRANSMITTING COATING IN FIRM CONTACT WITH THE OPPOSING FACE OF SAID LAYER, THE IMPROVEMENT CONSISTING ESSENTIALLY OF A PROTECTIVE TRANSPARENT FILM OF A COPOLYMER CONSISTING ESSENTIALLY OF AN OLEFIN CONTAINING FROM 2 TO 3 CARBON ATOMS PER MOLECULE 